1. Field of the Invention
This invention relates to polyurethane compositions and more particularly relates to polyurethane compositions which are heat curable to yield hard, impact-resistant articles which are useful in glazing applications.
2. Brief Description of the Prior Art
Hard, rigid, plastic materials such as polycarbonate and acrylics have been gaining considerable favor as substitutes for glass. The plastics are lighter in weight, more flexible and have sufficient strength for many applications in which glass is used. An example of where hard, rigid plastics are being used as a substitute for glass is in aircraft glazing where both polycarbonate and acrylics have enjoyed widespread acceptance. Unfortunately, there are some serious disadvantages associated with both polycarbonates and acrylics. Polycarbonates scratch easily and if directly exposed soon become difficult to view through. Acrylics, although not as scratchable as polycarbonates, do not have the physical properties of the polycarbonates such as heat distortion temperature and impact resistance.
The present invention overcomes many of these shortcomings of the plastics of the prior art and provides for a new hard plastic material which has a more desirable blend of physical properties than conventional polycarbonate and acrylics. The plastics of the present invention are polyurethanes prepared from cycloaliphatic polyisocyanates, low molecular weight active hydrogen-containing materials and optionally polymeric polycarbonate diols; the weight percentages of urethane moieties, cyclohexyl moieties and the extent of crosslinking in the polymer is carefully controlled so as to properly balance the physical properties.